1. Field of the Invention
This invention relates to a method for producing a molded crystalline resin article and especially a method of crystallizing the molded resin article.
2. Detailed Description of the Related Arts
Exterior equipment such as side moldings and bumper corners have conventionally been required to be highly glossy for the purpose of exhibiting an appearance of high quality. For this type of exterior equipment, those that are obtained by molding resins are generally employed because they are readily incorporated into automobiles and have satisfactory pliability and strength.
Examples of such molded resin articles are those having a double-layered structure consisting of a core layer and a skin layer (Japanese Laid-Open Publication No. 8-127107).
A core layer is present as a composite material having suitable pliability and rigidity which is obtained by mixing a polypropylene resin with a rubber component. A skin layer is made from a material obtained by mixing a crystalline polypropylene resin having a high Rockwell hardness with additives such as a colorant, and exhibits a satisfactory appearance, high gloss, metal-like surface and an anti-abrasion property.
A molded resin article which has the two layers described above is produced by a two-color molding method in which the starting material for the core layer and the starting material for the skin layer are injected simultaneously into the cavity of a mold and then cooled.
However, the molded resin article described above has conventionally been produced by heating the mold to a high temperature during injection of the resin and then cooling the mold to a low temperature upon cooling the resin in order to obtain the anti-abrasion property. In such a procedure, a prolonged period to raise or lower the temperature of the mold is required, and an enormous energy consumption is associated therewith.
Thus, it has been difficult to produce a large amount of molded resin articles by molding resins at a low cost.
Accordingly, for the purpose of improving producibility, it was proposed that the temperature of the mold be kept at a temperature as low as about 30xc2x0 C. and then the molded crystalline resin article is cooled rapidly. Nevertheless, such procedure undergoes insufficient crystallization of the skin layer of the resin article especially at its surface, resulting in a poor abrasion resistance.
In view of the hitherto problems described above, the object of this invention is to provide a method for producing a molded crystalline resin article which raises the degree of the crystallization to a high degree and further, to mass-produce the molded crystalline resin articles.
This invention provides a method for producing a molded crystalline resin article comprising molding a molded resin article from a thermoplastic resin containing a crystalline resin followed by performing a heating process to obtain a molded crystalline resin article having a surface region with a crystallization index of 0.9 or higher when determined by IR spectrometry.
In the present invention, a crystallization index means a value based on which the crystallization degree of a molded resin article can be evaluated. The crystallization index can be obtained by determining IR spectrum and then calculating the ratio of the peak intensity of the crystal portion to the peak intensity of the non-crystal portion.
In the present invention, a thermoplastic resin containing a crystalline resin means a thermoplastic resin that is partly or entirely formed of a crystalline resin. Thus, all or part of the thermoplastic resin is crystalline resin. In order to obtain a surface layer with a crystalline index of 0.9 or more, the crystalline resin should be contained in the thermoplastic resin which constitutes the surface layer.
Specifically, it is favorable that the crystalline resin is present in the thermoplastic resin in a range of 20-100 wt. % since this weight percent range enables formation of a surface layer with a crystalline index of 0.9 or more. On the other hand, where less than 20 wt. % crystalline resin is present, a crystalline index or 0.9 or more may not be obtained since the amount of the crystalline resin present is too low.
Further, it is more favorable that the crystalline resin is present in the thermoplastic resin in a range of 30-100 wt. %. With this range, a surface layer with a crystalline index of 0.9 or more can be more easily formed.
The surface layer of the molded crystalline resin article is readily subjected to the effect of atmospheric temperature since it is located on the surface of the molded crystalline resin article. Therefore, on the surface, the temperature is lowered more quickly and the crystallization tends to proceed insufficiently when compared with the inner portion of the resin article.
To solve this problem, the resin article is heated after being molded in the present invention. Therefore, the heating serves to promote further crystallization in the surface layer of the molded resin article.
The molded resin article is heated so that the crystallization index of the surface layer becomes 0.9 or higher. Accordingly, the crystallization of the surface layer of the molded resin article can be proceeded sufficiently. As a result, the surface layer of the molded resin article is brought to the state where the crystallization is almost saturated. Therefore, a highly crystalline molded resin article can be obtained. The upper limit of the crystallization index may vary depending on the crystallization degree specific to a certain crystalline resin, and, for example, that of a polypropylene resin is about 1.03.
Since the surface layer is highly crystalline, it exhibits excellent abrasion resistance, excellent appearance, a high gloss and a metal-like surface.
In addition, since the heating process serves to establish a state where the crystallization is almost saturated, a subsequent process in which the article is cooled gradually to proceed the crystallization is not required. Accordingly, the cooling after the heating process can be performed quickly.
Therefore, a highly crystalline molded resin article can be produced in a short time at a high efficiency. In addition, the energy consumed during the production can be reduced since the mold employed to mold the resin article can be kept at a low temperature.
On the other hand, when the crystallization index of the surface layer after the heating process is less than 0.9, the crystallization of the surface layer is insufficient, resulting in a reduced abrasion resistance and a poor appearance.
According to the present invention, the crystallization degree of a molded crystalline resin article can be raised to a higher degree, and a method for producing a molded crystalline resin article on a large scale is provided.